Why rooflights mist between the panes

The glass in a modern rooflight is a sealed unit: two or three panes held apart by a spacer bar around the edge, with a gap between them. The edge is sealed twice. A primary seal, usually a butyl strip, sits against the glass and keeps moisture out. A secondary seal of polysulphide, silicone or polyurethane sits outside the spacer and holds the whole unit together mechanically.

The spacer bar is hollow and filled with a desiccant, a drying agent that absorbs the small amount of moisture present at manufacture and any that creeps in over the years. The gap itself is filled with dry air or, in better units, argon, which conducts heat more slowly than air. A low emissivity coating on one pane faces the gap, which is how the unit cuts heat loss.

All of that depends on the seal staying intact. Desiccant has a limited capacity, and when moisture gets in faster than it can absorb, it saturates. From then on, water vapour sits in the gap and turns to droplets on the glass when the temperature falls. The glass is not cracked and the frame is not necessarily leaking, which is why a misted unit can look fine from the roof and be a mess from the room.

A sealed unit in a wall is stressed by temperature and wind. One in a roof gets all of that, plus a few extra loads.

Heat from above

A rooflight faces the sky, so on a bright day the outer pane takes full sun while the inner pane stays near room temperature. The temperature difference makes the unit flex, and a flexing seal opens microscopic gaps over time. Dark frames and solar control coatings can raise the temperature further.

Standing water

On a low-pitch or flat roof, rain does not run off a pane as quickly. A rooflight bedded in a frame that holds water around the unit's edge keeps the secondary seal wet for longer than it was designed for. Sealant that sits in water ages faster.

Ultraviolet light

Seal compounds are tested against UV, but the roof gets more of it than any vertical wall. Over the lifetime of a unit, the edges of the sealant can chalk and harden.

Pressure changes

The gas inside a sealed unit expands when it is warm and contracts when it is cold. A big difference in altitude or air pressure between factory and site can add stress. Most units cope, but the seal is working every day of its life.

None of that is a flaw in the idea of a sealed unit. It means a seal on a roof has less margin than one in a wall, and any weakness in manufacture, installation or the frame shows up sooner.

When a unit fails earlier than it should, one or more of these is usually behind it.

• Manufacturing defect. Poor sealant coverage, a gap at a spacer corner or insufficient desiccant can cause failure within a few years. This is a manufacturer's issue, and it is covered by their own warranty if the unit is within it.
• Drainage and ventilation of the rebate. The glass should sit in a frame with a small drained and ventilated gap beneath it. If that gap clogs, or the frame is sealed solid, water collects against the edge seal.
• Incorrect glazing packers. Glass supported at the wrong points, or without spacers where they are needed, flexes more and loads the seal.
• Age. Sealed units are not permanent. Nothing here is a fault; every unit has a lifespan, and the sealant is usually the part that runs out first.
• Cleaning damage. Aggressive solvents or sharp tools at the edge can attack the sealant. Gentle washing with a soft brush and water is enough for most glass.

If the rooflight is still within its manufacturer's warranty, photograph the misting, keep the invoice, and tell whoever fitted it. A failed unit that is still inside the period is likely to be a replacement under warranty, not a cost. Manufacturers can usually tell from the date stamp on the spacer bar how old a unit is.

Not every damp pane is a failed seal. Where the water sits tells you most of what you need to know, and a short check will save a wrong guess.

Condensation on the outer face is a surprise to many owners, but it is a sign the glass is doing its job: very little heat is escaping to warm it. Condensation on the room face is more of a prompt to look at humidity, extraction and how well the lining is insulated. Our answer on whether rooflights can cause condensation goes through those cases.

A misted unit usually shows as a cloudy film, droplets or a pale mineral stain trailing from one edge. It may fill and clear with the weather, which tempts people to wait and see. It does not cure itself, because the seal has not healed.

A misted rooflight is mainly a comfort and appearance problem at first, and a thermal one as it gets worse. Looking up through the glass gets blurry, and the brightness of the room can drop where a lantern is the main light source. In a room with a lining finished to a good standard, a stained pane at the centre of the ceiling is hard to ignore.

On the thermal side, once the gap has air and moisture instead of argon, the unit insulates less. The low emissivity coating can also be damaged by the moisture, which reduces its effect. How much heat is lost depends on the unit and how far the failure has gone, so there is no single figure to quote, but the direction is always the same. We look at that in does a misted rooflight lose heat.

Misting is not a safety problem on its own, because the glass has not weakened. If the glass is cracked as well, that is a different matter, and it is covered in signs a rooflight needs replacing.

There are services that drill small holes in the glass, clean the inside and fit vents. They deal with the cloudiness, but they do not restore the seal or the gas, so a treated unit performs as a vented single-skin pane at best. On a rooflight, drilling a hole in a pane that may sit overhead is also a poor idea for the structure of the glass, and a laminated inner pane cannot be treated that way at all.

In practice the options come down to two. A clouded unit can be replaced, or the whole rooflight can be. We go through the first in can misted rooflight glass be cleared, and the choice between them is the next section.

If the frame is sound, the usual answer is to replace the sealed unit alone. A new unit is made to the measured size of the old one, the old glass is removed, and the new one is bedded in. That is not notifiable work under the Building Regulations, because the frame stays. It suits a frame in good condition with no rot, no corrosion and no failed seals around its edges. Our guide to replacing just the glass in a rooflight explains the process and how the measuring works.

Replacing the whole rooflight is the right call when the frame has aged along with the glass. A timber frame that has softened, an aluminium frame with failing corner seals, an old unit with no thermal break, or a kerb that has moved can all make a new unit pointless. A whole replacement is notifiable: the new window is a controlled fitting and has to meet Part L, and we handle the Building Control notification. More on what that involves is on Part L and replacement rooflights, though that is the energy side; the other half of the decision is what the room looks like afterwards.

A new unit is also the moment to improve the glass. Modern sealed units are warmer than ones fitted a couple of decades ago, with better coatings, argon filling and warm-edge spacers that cut the cold bridge at the perimeter, where condensation tends to start. Laminated inner panes are a sensible choice overhead. Done at the replacement stage, these cost little in comparison with the access and labour.

The rooflight is the part of the job you see on the roof, but the room is where the disturbance lands. Taking out an old unit and frame sends dust and debris down the opening, and the edge of the lining takes knocks. In a kitchen or a bedroom with a good ceiling, this is where care counts.

Before anything comes out, the floor and furniture are covered and a dust screen goes up. If it is a unit swap, the lining stays untouched and only the glazing bead is lifted. If it is a whole replacement, the lining may need to be stripped or re-formed, particularly if stains at the sill show it was cold. That is also the chance to add the insulation it never had, and to splay it if it was straight. Our guide to replacing a rooflight without damaging the ceiling covers the order the work goes in, and how a lath ceiling or a skimmed one is protected differently.

Every quote of ours comes with a finish schedule, which records the lining, the plaster, the paint line and how dust is managed, so what happens in the room is agreed before it starts.

No seal lasts forever, but a few choices help a new unit last as long as it can.

• Choose a unit with a warm-edge spacer and a good-quality secondary seal, and ask what the manufacturer's warranty on the sealed unit covers and for how long.
• Make sure the frame is drained and ventilated at the glazing rebate, and that nothing blocks it during fitting or painting.
• Clean with water and a soft brush or sponge, and avoid solvents or sharp edges at the unit's perimeter.
• On a flat roof, check that the kerb and fall let water leave the glass quickly rather than sit around the edge.

Unit warranties are normally written by the glass maker, not the installer, and are often 10 years or thereabouts, but check the paperwork rather than trusting memory. Keep the order confirmation and the date of fitting with your finish schedule, so a claim is simple if one is ever needed.

If a rooflight in your house has misted, the first step is to find out whether the frame is worth keeping. A survey answers that in one visit: we look at the unit, the frame, the flashing and the lining, and tell you whether a new sealed unit will do or the whole rooflight should go. You get a written quote either way. Our misted and failed rooflight replacement page sets out the options, and you can ask for a quote with a photo of the pane and the room to speed things up.